It is a common problem to measure the velocity of a moving object, where the object is observed through a probing field. An apparatus is used for emitting a pulsed field that is scattered or reflected by the moving object, and then received by a receiving transducer or antenna. Repeating the experiment yields signals that can be used in an algorithm for determining the velocity of the object.
Equipment of this kind is used in diagnostic medical ultrasound systems for measuring the velocity of blood flow non-invasively. Here a series of ultrasound pulses are emitted, and the scattered signal from the blood is measured as described by Baker (1970). The ultrasound signals will be displaced or offset in time according to the blood flow velocity and speed of sound, and the movement is detected by the apparatus, and the velocity is calculated from the time between pulses and the movement. The velocity can be displayed either as the velocity distribution as a function of time (spectrogram), or as a single velocity over time. This technique can also be used to display velocity images. Here the ultrasound beam is emitted a number of times in one distinct direction, and the velocities along that direction are found by dividing the received signal into segments and finding the velocities for the different depths. The direction is then changed. The measurement procedure is then repeated and the velocities found along these other directions. An image of velocity is then made, and continuously updated over time. The velocity can be found through an autocorrelation approach as described by Kasai et al. (1985) and Namekawa et al. (1982). Another technique is to use cross-correlation as described by Bonnefous et al. (1986). A general description of the systems can be found in Jensen (1996). In these methods the standard techniques for generating homogeneous and highly focused fields are used in order to obtain a highly focused image with a uniform appearance for all depths.
Radar systems also use the pulse principle for estimating velocity of a moving object. A series of radar pulses is emitted and the received signals are measured. The signals from a specific distance are compared and the velocity is calculated from the movement of the object between pulses, the speed of light, and the time between pulse emissions. This is, e.g., used for finding the velocity of airplanes, missiles, or ships, as described by Skolnik (1980).
The pulse movement principle has also been employed in sonar for finding the velocity of different objects. This is done by the same methods as mentioned above for medical ultrasound scanners with appropriate adaptations. One problem with all these velocity estimation techniques is that only the velocity component in the beam direction, i.e. towards or away from the transducer, can be found. Any velocity perpendicular to the beam propagation direction can not measured. A number of approaches have sought to remedy this in diagnostic medical ultrasound. Two consecutive ultrasound images are measured in the speckle tracking approach as described by Trahey et al. (1987). The movement of a region in a regional pattern from the one image to the next is found through two-dimensional cross-correlation, and the velocity vector for the region is determined from the displacement of the region and the time between the images. The technique needs two images, which makes data acquisition slow, and precludes the use of averaging. The image acquisition also makes this technique difficult to use for full three-dimensional velocity estimation. The two-dimensional correlation necessitates a high number of calculations, and erroneous velocities can evolve due to false maxima in the correlation function.
Another approach is to use two transducers or apertures emitting two beams crossing each other in the region of interest, whereby the velocity can be found in two independent directions. The velocity vector can then be found through a triangulation scheme. The variance and hence the accuracy of the transverse component of the velocity is affected by the angle between the two beams. The angle will be small at large depths in tissue, and a small angle will give a high variance, i.e. a low accuracy. The use of two transducers or a single large array also makes probing between the ribs of a person difficult, and can result in loss of contact for one of the transducers.
EP 0 638 285 A1 discloses an ultrasonic diagnostic apparatus which, based on the Doppler bandwidth, estimates amplitude and direction independently. The method is in principle based on the idea originally presented by Newhouse et al., "Study of vector flow estimation with transverse Doppler", IEEE Ultrasonics Symposium, pp. 1259-1263, (1991). U.S. Pat. No. 4,979,513, Sakai et al. discloses an ultrasonic diagnostic apparatus which estimates phase changes across the face of the transducer aperture, and the lateral velocity is found from the complex time demodulated radio-frequency signal.